The present invention relates to a retrofocus lens system suitable for a projection optical system which requires a long back focal distance in comparison with a focal distance and a projection display apparatus incorporating the retrofocus lens system.
FIG. 13 is a schematic diagram showing a configuration of an optical system of a conventional projection display apparatus (a liquid crystal projector). As shown in FIG. 13, the projection display apparatus 300 comprises a light source 1 which includes a lamp 120 and a reflecting mirror 130 and emits approximately parallel illuminating light 2, dichroic mirrors 3B and 3G, and light reflection mirrors 4a, 4b, and 4c. The projection display apparatus 300 further comprises a transmissive liquid crystal panel 5R for displaying a red image, a transmissive liquid crystal panel 5G for displaying a green image, a transmissive liquid crystal panel 5B for displaying a blue image, a dichroic prism 6 which outputs combined light 20 of red (R), green (G), and blue (B) by reflecting the red light 2R and the blue light 2B and passing the green light 2G, and a projection lens 7 for projecting incident light 20 onto a screen 8 with a magnification. In the figure, a reference numeral 200 denotes a housing.
The dichroic mirror 3B receives the light 2 emitted from the light source 1, reflects the blue light 2B, and allows the red light 2R and the green light 2G to pass. The blue light 2B reflected from the dichroic mirror 3B is reflected by the mirror 4b, passes the liquid crystal panel 5B, and then enters the dichroic prism 6. The dichroic mirror 3G reflects the green light 2G that has passed the dichroic mirror 3B and allows the red light 2R to pass. The green light 2G reflected from the dichroic mirror 3G passes the liquid crystal panel 5G and enters the dichroic prism 6. The red light 2R that has passed the dichroic mirror 3B is reflected by the mirrors 4a and 4c, passes the liquid crystal panel 5R, and enters the dichroic prism 6. The dichroic prism 6 sends out the combined light 20 of the incident red light 2R, green light 2G, and blue light 2B toward the projection lens 7. The projection lens 7 projects the combined light 20 onto the screen 8 with a magnification.
In the above-mentioned projection display apparatus, the thick dichroic prism 6 must be disposed between the projection lens 7 and the liquid crystal panels 5R, 5G, and 5B functioning as light valve components, which are picture sources, so that the projection lens 7 requires a long back focal distance.
If the above-mentioned projection display apparatus is used in a rear projector (a rear projection display apparatus), it is preferable that the distance between the projection lens 7 and the screen 8 should be short (that is, the projection lens 7 should have a wide angle of view) in order to reduce the outer dimensions of the apparatus.
Because the spectral transmittance, polarization generation characteristics, and reflectivity of the dichroic prism 6 greatly vary with the incident angle of the light, the design is provided so that the illuminating light striking the liquid crystal panels 5R, 5G, and 5B become approximately parallel light (that is, telecentric illumination is provided). In this case, the light striking the projection lens 7 is approximately parallel light. If this type of optical system uses a conventional wide-angle projection lens having a short back focal distance, the light that passes the perimeter of the liquid crystal panels 5R, 5G, and 5B and then strikes the projection lens 7 is extremely reduced, causing the projection image to become dark at the perimeter of the screen 8. Accordingly, it is desired that the apparatus be configured to make the principal ray of the light coming from the individual points of the picture source approximately parallel to the optical axis of the projection lens 7 (telecentric configuration). This configuration requires such a projection lens that the distance between the projection lens 7 and the position of the pupil is sufficiently greater than the focal distance.
As has been described above, a projection lens used in a projection display apparatus is required to satisfy the basic specifications associated with (1) a wide angle of view, (2) a long back focal distance, and (3) telecentric characteristics on the image display component side. The projection lens of the projection display apparatus is also required to have basic aberration characteristics (4) to (7) described below.
(4) Low chromatic aberration: The chromatic aberration of magnification must be representatively kept around the pixel pitch or preferably suppressed below a half of the pixel pitch, so that the projection magnification difference in primary-color pixels of the projection image is sufficiently reduced. When an ultra-high pressure mercury lamp is used for the illumination light source, the light output may contain a strong spectrum at a wavelength shorter than the inherent spectral wavelength of the blue light, which is on the order of 450 nm to 470 nm, or in the proximity of the mercury g-line (436 nm). In such a situation, it is necessary that the chromatic aberration of magnification for such emission line spectral component be corrected in consideration of the chromatic aberration of magnification for red spectral components so as to suppress violet flare components. It is also necessary to control the longitudinal chromatic aberration so that the focal points for primary colors are placed at the same point.
(5) Low distortion: Since a wide-angle lens for the rear projector projects a rectangular projection image inside the frame of the projection screen, the distortion around the perimeter of the screen often stands out. Accordingly, the deviation of a pixel from its ideal point resulting from the distortion must be representatively restricted to the order of the pixel pitch. In rear projectors for use in CAD, multi-vision projectors that increase the number of pixels by arranging unit screens formed by rear projection and the like, it is required to control the distortion so that the absolute deviation from an ideal point is restricted to or below a half of the pixel pitch.
(6) Wide operating temperature range: The projection lens should be designed to maintain desired optical characteristics over a wide temperature range, so that the lens can be used in a wide temperature environment in which the projector is placed and can endure the heat generated by the illumination lamp. To provide the wide operating temperature range, it is preferable that the projection lens be configured only by glass lenses. In comparison with plastic materials lens, glass lenses generally exhibit small variations in expansion and refractive index with temperature variations, which favors the maintenance of stable optical characteristics. However, if an aspheric surface is used to correct aberrations, glass lenses have a cost disadvantage. The lens system of the present invention corrects aberrations with plastic aspheric lenses and implements a projection lens with small defocusing due to temperature variations.
(7) High resolution: To project an original image produced by a light valve component having many pixels on the order of one million pixels at a high density, which has been increasingly developed in recent years, with a magnification, a projection lens having a high resolution matching the fine pixel structure of the light valve is needed. To ensure the high resolution of the projection lens, the chromatic aberration and distortion described above, and other axial aberrations and off-axis aberrations must be sufficiently corrected.
It is an object of the present invention to provide a retrofocus lens system which has a long back focal distance in comparison with a focal distance and telecentric characteristics on the picture source side and allows wide-angle projection, and a projection display apparatus utilizing the retrofocus lens system.
According to the present invention, a retrofocus lens system comprises in order from a large conjugate side toward a small conjugate side: a first lens group having a negative refracting power; a second lens group having a positive refracting power; and a third lens group having a positive refracting power. The first lens group includes in order from the large conjugate side toward the small conjugate side, a first lens having an aspheric surface, a meniscus second lens having a negative refracting power and having a convex surface on the large conjugate side, a meniscus third lens having a negative refracting power and having a convex surface on the large conjugate side, and a meniscus fourth lens having a negative refracting power and having a convex surface on the small conjugate side. The second lens group includes in order from the large conjugate side toward the small conjugate side, a fifth lens having a positive refracting power, and a sixth lens joined to the fifth lens. The third lens group includes in order from the large conjugate side toward the small conjugate side, a meniscus seventh lens having a positive refracting power and having a convex surface on the small conjugate side, a biconcave eighth lens, a ninth lens joined to the eighth lens and having a positive refracting power, a biconvex tenth lens, a biconvex eleventh lens, and a twelfth lens having an aspheric surface. The retrofocus lens system satisfies the following expressions:
0.8 less than f2/f3 less than 1.5xe2x80x83xe2x80x83(1)
1.6 less than |f1|/f less than 2.4xe2x80x83xe2x80x83(2)
|f4|/f greater than 30xe2x80x83xe2x80x83(3)
f5/f greater than 6xe2x80x83xe2x80x83(4)
where f is a focal distance of the whole lens system, f2 is a focal distance of the second lens group, f3 is a focal distance of the third lens group, |f1| is an absolute value of a focal distance of the first lens group, |f4| is an absolute value of an axial focal distance of the first lens, and f5 is an axial focal distance of the twelfth lens.
The retrofocus lens system satisfying the expressions (1) to (4) can provide an advantage that off-axis aberrations can be appropriately corrected while the long back focal distance and the telecentric performance are maintained. In addition, by restricting the axial power of the first lens and the twelfth lens, both of which comprise a plastic material, to a small value, the retrofocus lens system can provide another advantage that it can be used over a wide temperature range while defocusing and degradation in the resolution due to the temperature change can be eliminated.
The retrofocus lens system may further comprise a stop disposed between the second lens group and the third lens group; wherein the retrofocus lens system satisfies the following expression:
|EXP|/f greater than 50xe2x80x83xe2x80x83(5)
where |EXP| is an absolute value of a distance from an image surface on the small conjugate side to a pupil surface on the small conjugate side in the whole lens system.
The retrofocus lens system satisfying the expression (5) can provide an arbitrary choice of the brightness and the focussing performance in accordance with an illumination system up to the projection lens. Further, the retrofocus lens system can project light modulated by the light valve that is subject to the telecentric illumination with a favorable peripheral relative illumination, by increasing the pupil distance on the light valve side.
The retrofocus lens system may satisfy the following expression:
xe2x80x83BFL/f greater than 2xe2x80x83xe2x80x83(6)
where BFL is a back focal distance of the whole lens system.
The retrofocus lens system satisfying the expression (6) can provide a long back focal distance so that a required air spacing can be secured between the light valve and the retrofocus lens system in order to mount a thick prism element, a cover glass which protects the front surface of the light valve and a projection system in an appropriate manner.
The retrofocus lens system may satisfy the following expressions:
15 less than xcexd4 less than 30xe2x80x83xe2x80x83(7)
15 less than xcexd5 less than 30xe2x80x83xe2x80x83(8)
40 less than xcexd7 less than 100xe2x80x83xe2x80x83(9)
15 less than xcexd8 less than 32xe2x80x83xe2x80x83(10)
20 less than xcexd9 less than 50xe2x80x83xe2x80x83(11)
70 less than xcexd10 less than 100xe2x80x83xe2x80x83(12)
where xcexd4 is an Abbe number at d-line of a glass material forming the fourth lens, xcexd5 is an Abbe number at d-line of a glass material forming the fifth lens, xcexd7 is an Abbe number at d-line of a glass material forming the seventh lens, xcexd8 is an Abbe number at d-line of a glass material forming the eighth lens, xcexd9 is an Abbe number at d-line of a glass material forming the ninth lens, and xcexd10 is an Abbe number at d-line of a glass material forming the tenth lens.
The retrofocus lens system satisfying the expressions (7) to (12) can provide appropriate chromatic aberrations (longitudinal chromatic aberration and chromatic aberration of magnification) to suppress a color shift of primary color images which is projected with a magnification and to achieve a high resolution.
The retrofocus lens system may satisfy the following expressions:
dPgFm=PgFma(xcexdd)xe2x88x92PgFn(xcexdd)
0.008 less than dPgF4 less than 0.03xe2x80x83xe2x80x83(13)
0.01 less than dPgF5 less than 0.025xe2x80x83xe2x80x83(14)
0.01 less than dPgF8 less than 0.02xe2x80x83xe2x80x83(15)
0.03 less than dPgF10 less than 0.055xe2x80x83xe2x80x83(16)
where dPgFm denotes a parameter representing anomalous dispersive properties of the glass material which forms the m-th lens, m being equal to 4, 5, 8 or 10, PgFn(xcexdd) denotes a straight line representing a normal partial dispersion ratio in a coordinate system that has the abscissa indicating the Abbe number xcexdd at d-line and the ordinate indicating a partial dispersion ratio PgF from F-line to g-line, and PgFma(xcexdd) denotes an anomalous partial dispersion ratio of the glass material which forms the m-th lens having the Abbe number xcexdd at d-line.
The retrofocus lens system satisfying the expressions (13) to (16) can provide a satisfactory projection image which is relatively free from flare components in a projection display apparatus including a light source of a short wavelength emission line spectrum.
The retrofocus lens system may satisfy the following expressions:
dPgFm=PgFma(xcexdd)xe2x88x92PgFn(xcexdd)
xe2x88x920.01 less than dPgF7 less than 0.045xe2x80x83xe2x80x83(17)
xe2x88x920.015 less than dPgF9 less than 0.02xe2x80x83xe2x80x83(18)
where dPgFm denotes a parameter representing anomalous dispersive properties of the glass material which forms the m-th lens, m being equal to 7 or 9, PgFn(xcexdd) denotes a straight line representing a normal partial dispersion ratio in a coordinate system that has the abscissa indicating the Abbe number xcexdd at d-line and the ordinate indicating a partial dispersion ratio PgF from F-line to g-line, and PgFma(xcexdd) denotes an anomalous partial dispersion ratio of the glass material which forms the m-th lens having the Abbe number xcexdd at d-line.
The retrofocus lens system satisfying the expressions (17) and (18) can provide a satisfactory projection image which is relatively free from flare components in a projection display apparatus including a light source of a short wavelength spectrum.
According to another aspect of the present invention, a projection display apparatus comprises: a light source for emitting light; a light valve for two-dimensionally modulating the light from the light source; and the above-mentioned retrofocus lens system for projecting with a magnification the light modulated by the light valve.